The radio common carrier (RCC) pager industry began in the mid-1940s when the Federal Communications Commission (FCC) began allocating frequencies for local communication services. Several different types of electronic paging systems have been developed and several different types of systems are in widespread use. The need for standards is well recognized and several different competing standards have been developed.
One of the most commonly used formats is the POCSAG coding format. This format was developed by the British Post Office "Post Office Codes Standardization Advisory Group". Other existing formats are the GOLAY sequential code format (sometimes called GSC) which is often attributed to Motorola, the D1 and D2 formats which were developed by the NEC Corporation, and the FM subcarrier protocol which is shown in U.S. Pat. No. 4,713,808 (Gaskill).
No single protocol has been able to capture universal or near universal acceptance. The reason for this is that each protocol generally has advantages and disadvantages. For example, the GOLAY protocol requires more air time per message than does the POCSAG protocol; however, the POCSAG protocol does not support voice paging, whereas the GOLAY does support most voice paging. The GOLAY protocol provides a greater degree of error correction because the messages are encoded with more redundancy than are the POCSAG messages. However, this added redundancy is the reason that the GOLAY protocol requires more airtime. Thus, in each application the advantages and disadvantages of each protocol must be balanced to achieve optimum performance.
In addition to using various possible transmission protocols, pagers may operate at any one of a variety of frequencies. The Federal Communications Commission (FCC) has set aside several parts of the radio frequency spectrum for use by RCC radio paging systems. More recently, the Federal Communication Corporation began allocating frequencies in the 900 MHz for use by nationwide paging systems. The frequencies set aside by the FCC are:
a) 30-50 MHz (low band VHF) PA1 b) 148-174 MHz (high band VHF) PA1 c) 450-512 MHz (UHF) PA1 d) 900-950 MHz (900 MHz space band) PA1 f) 88-108 MHz (FM-SCA or FM-Subcarrier or Subcarrier Communication Authorization )
In order to accommodate the various protocols, sophisticated paging devices have been developed which can accommodate multiple protocols. For example, U.S. Pat. No. 4,518,961 (Davis), which is assigned to Motorola, describes a battery-powered pager which is capable of processing detected signals in accordance with plurality of de-coding schemes. This pager includes a micro-processor which detects which coding scheme is being used, and then de-codes the message in accordance with the appropriate protocol.
Some pagers can handle signals on more than one frequency. For example, an FM-subcarrier (SCAG) pager being marketed by the CUE Paging Corporation (formerly called DiversiCom) scans the FM frequencies to find a frequency which has an FM-SCA signal of the type broadcast the CUE system. When the pager determines an appropriate signal on one of the FM frequencies, it stays tuned to that frequency and receives the paging message.
A system developed by AT&E Corporation, some aspects of which are shown in U.S. Pat. Nos. 4,713,808 and 4,897,835, transmits using a time slot protocol on an FM subcarrier of commercial FM stations. In the system shown in the above patents, the same message is transmitted on a number of different stations, the transmissions being slightly offset one from another, so that if when a receiver turns on in its assigned time slot, it misses a message, in a very short time it can tune to another station, then to still another station, etc to try to receive the message.
None of the prior art systems can receive messages which are transmitted both on an FM-SCA system and messages transmitted over an RCC system using the POCSAG or the GOLAY protocol. Likewise, none of the systems can receive signals on the VHF-UHF or 900 mega-hertz band and also detect signals in the FM-SCA band.
One of the problems encountered by systems using FM-SCA signals that are transmitted over commercial FM stations is that certain areas are "dead-spots" where signals can not be received. The cost of installing additional FM transmitters which operate using the FM-SCA system to cover dead spots is generally prohibitive, hence, it is desirable to utilize existing or new RCC systems in areas where transmissions dead spots occur. However, no existing pagers can accept both FM-SCA signals and RCC signals.
The present invention provides a system which includes a plurality of transmitters operating at different frequencies and using different protocols and an improved pager which can automatically accept both RCC and FM-SCA signals.